Semitransparent ZnO-based UV-active solar cells: Analysis of electrical loss mechanisms

Three different ZnO-based diodes are compared that can be employed as semitransparent ultraviolet (UV)-active solar cells: a Schottky diode using platinum oxide as front contact, a p+n diode with magnetron-sputtered nickel oxide and a pn diode with a pulsed laser deposited NiO front contact. The UV conversion efficiencies are 4.1% for the Schottky diodes and 3.1% for the NiO-based cells. In the NiO-based structures, a strong deformation of the current–voltage characteristics under white light illumination (one sun) is observed, leading to reduced open-circuit voltages. Measurements of the external quantum efficiency with and without simultaneous white-light illumination reveal that also the collected photocurrent in these devices types is significantly reduced under strong illumination. It is shown that the magnitude of both the injected current and the recombination current of photogenerated carriers is increased in this state. A model is proposed that explains both effects within the framework of an optically activated recombination channel at the NiO/ZnO interface.